okay, here’s what seems to be happening:
if i choose “Export/QT conversion” in the QT format, it takes an extreme amount of time to even encode a 3-minute clip. in fact, as it renders, the time remaining keeps shooting up to over an hour…so i cancelled that process.
if i choose “Export/QT movie/current settings/self-contained” (the one that seems the mostly likely correct candidate), it renders quickly, but does not play if i “hide” the source file on another disc (as if it had been deleted). but as soon as i reconnect with the source file, this new one plays fine (though it’s defeating the purpose, as i want to get rid of the large source file).
thanks for helping out a bonehead!